1. Field of the Invention
The present invention relates to a power management device, a control system, and a control method and, more particularly, to a power management device for managing power supplies, a control system having the power management device and an engine control device, and a method for controlling cranking of a starter.
2. Description of the Relevant Art
In recent years, in order to respond to user needs, comfort and convenience of a vehicle have been rapidly progressing. A push start system developed for improving the convenience when starting an engine is exemplified (see Japanese Registered Utility Model No. 3060902, Japanese Patent Application Laid-Open Publication No. 2002-122058, and Japanese Patent Application Laid-Open Publication No. 2005-248859, for example).
In the push start system, it is unnecessary to insert an engine key into a key cylinder to turn the key to an ST (Starter) position, differently from a mechanical start system, and the engine is started with a push of a button.
FIG. 1 is a block diagram schematically showing a push start system. Reference numeral 1 in FIG. 1 represents a battery. From the battery 1, power is supplied through an ACC relay 2 to ACC (Accessory) units Ua1, Ua2, . . . , and through an IG relay 3 to IG (Ignition) units Ub1, Ub2, . . . . The ACC relay 2 and the IG relay 3 are turned on by the passage of electric current through coils L2 and L3, respectively.
To a starter motor 4, power is supplied through a motor relay 5 from the battery 1. Turning-on/-off of the motor relay 5 is controlled by the passage of electric current through a coil L5. When power is supplied to the coil L5, the motor relay 5 is turned on, the starter motor 4 is actuated, and an engine is started.
Power from the battery 1 is supplied to the coil L5 in cases where an ST relay 7 is turned on while a safety switch 6 is in an ON state. The ST relay 7 is turned on by the passage of electric current through a coil L7. The safety switch 6 is in the ON state in cases where a selector lever is in a P (Parking) position or an N (Neutral) position, or a clutch pedal has been pressed.
Applications of power to the coils L2, L3 and L7 which control turning-on/-off of the ACC relay 2, the IG relay 3, and the ST relay 7 are controlled by a power management device 8. To the power management device 8, the safety switch 6, a button switch 9 to be operated by a driver, and a brake switch (SW) 10 are connected.
When the button switch 9 is pressed while the safety switch 6 is in the ON state and the brake pedal is held down, the power management device 8 applies power to the coils L2, L3 and L7 so as to turn on the ACC relay 2, the IG relay 3 and the ST relay 7.
On the other hand, when the button switch 9 is pressed while the safety switch 6 is in an OFF state, or the brake pedal is not held down, without applying power to the coil L7, only the power condition is changed. For example, when the button switch 9 is pressed while the power condition is an OFF state, power is applied to the coil L2 so as to change the power condition to an ACC state. When the button switch 9 is pressed in the ACC state, power is applied to the coil L3 so as to change the power condition to an IG state. When the button switch 9 is pressed in the IG state, power to the coils L2 and L3 is cut off so as to change the power condition to the OFF state.
An engine control device 11 comprises a start management control unit 14 having an engine complete explosion determining function 12 and a starter hold control function 13. To the engine control device 11, an engine revolution sensor 15 is connected, and therefore, the engine control device 11 can grasp an engine speed. In addition, to the engine control device 11, the safety switch 6 and the ST relay 7 are connected.
When turning-on of the ST relay 7 is detected while the safety switch 6 is in the ON state, the engine control device 11 supplies power to an ST line Ln in order to hold cranking, and the cranking is held (cranking hold). Cranking hold is conducted since power is applied to the coil L7 by the power management device 8 only within a time period during which the button switch 9 has been pressed (i.e. power is not applied to the coil L7 when the button switch 9 is not pressed).
Thus, without the driver's continuing to press the button switch 9, it is possible to continue to drive the starter motor 4. The engine control device 11 determines whether the engine reached a complete explosion (i.e. whether the engine became able to keep revolutions under its own power or not) based on the engine speed or the like, and when determining that the engine reached the complete explosion, a power supply to the ST line Ln is terminated.
By the way, in recent years, owing to integration of vehicle control, it became possible to reduce electronic components to be mounted on a vehicle and further improve the dynamics of the vehicle. For example, an engine control ECU (Electronic Control Unit) and a transmission control ECU are combined into one, so as to conduct engine control at shifting gears. Moreover, lately, it became possible to exercise control over a vehicle such as power management (e.g. torque control) and heat management (e.g. heat control), leading to a higher-level vehicle control system (see Japanese Patent Application Laid-Open Publication No. 2003-329719 and Japanese Patent Application Laid-Open Publication No. 2004-136816, for example).
At present, the start management control function is located in the engine control device 11 as shown in FIG. 1, but it is considered that the function will be located in the power management device 8 in the future, by relocation of functions of vehicle control.
However, since the determination of complete explosion of the engine is influenced by engine conditions (e.g. the type of the engine), it is desired that the engine complete explosion determining function should be included in the engine control device 11 as it is without being moved to the power management device 8. That is because an engine speed to be a criterion of judgment of complete explosion differs depending on the type of the engine, for example.
Therefore, looking ahead, as shown in FIG. 2, it is expected that an engine control device 11a comprising a start management control unit 14a without a starter hold control function 13, and a power management device 8b comprising a start management control unit 14b having a starter hold control function 13b will make their appearances.
FIG. 3 is a block diagram schematically showing a push start system expected to appear in the future. Here, the same components as those of the push start system shown in FIG. 1 are similarly marked, and are not described below. Reference numeral 8b in FIG. 3 represents a power management device, which comprises a start management control unit 14b having a starter hold control function 13b. 
When a button switch 9 is pressed while a safety switch 6 is in an ON state and a brake pedal is held down, the power management device 8b applies power to coils L2, L3 and L7 so as to turn on an ACC relay 2, an IG relay 3 and an ST relay 7. The power management device 8b, differently from the conventional power management device 8, holds the application of power to the coil L7 so as to supply power to a starter motor 4 (cranking hold) until receiving a starter stop instruction signal indicating an instruction to terminate cranking hold, sent from an engine control device 11a when it is determined that an engine reached a complete explosion. In other words, the power management device 8b cuts off a power supply to the coil L7 so as to terminate cranking hold when receiving the starter stop instruction signal sent from the engine control device 11a. 
From the engine control device 11a, not only the starter stop instruction signal but also an engine revolution signal indicating an engine speed is sent to the power management device 8b. Therefore, the power management device 8b can judge whether the engine has reached a complete explosion or not based on the engine speed. As a result, the power management device 8b can cut off the power supply to the coil L7 so as to terminate cranking hold when the engine reached a complete explosion, even if the starter stop instruction signal could not be received (fail-safe processing).
By the way, since the power management device 8b conducts power management and the like to exercise control over the vehicle, the power management device 8b should not become inoperative. Even if a voltage of a battery 1 decreases below an operating voltage range of the power management device 8b (i.e. decreases below a lower limit of voltage required for a normal operation), the power management device 8b should not become inoperative. Then, it is necessary to allow the power management device 8b to have a boosting circuit so as to normally operate even when the voltage of the battery 1 decreases below the operating voltage range of the power management device 8b. 
However, since the engine control device 11a does not always have a boosting circuit, there is a risk that the power management device 8b may be unable to receive a starter stop instruction signal or an engine revolution signal when the voltage of the battery 1 decreases below an operating voltage range of the engine control device 11a so as to cause the engine control device 11a not to normally operate. One reason why the boosting circuit is not included in the engine control device 11a is an increase in cost.
When the power management device 8b cannot receive the starter stop instruction signal and the engine revolution signal, power is applied to the coil L7 for an indefinite time. Even though the engine has reached a complete explosion, cranking, is held. When the cranking is continued even though the engine has reached the complete explosion, there is a risk that a failure of the starter motor 4 may be caused, or that an unusual sound may be caused by the friction between a crankshaft and the starter (a gear), leading to user discomfort. Here, such friction is caused because the starter rotates the crankshaft till a complete explosion of the engine, but in reverse, the starter is rotated by the crankshaft after the complete explosion of the engine.
Moreover, when the voltage of the battery 1 decreases below the operating voltage range of the engine control device 11a, or when the engine control device 11a suffers a breakdown and runs away, the engine control device 11a cannot normally operate and becomes unable to conduct injection control, ignition control and the like, and therefore, there is no need to drive the starter. If the starter is continuously driven in such situation, a degradation speed of the battery 1 will be increased, resulting in shortening the life expectancy of the battery 1.